Minimum Brake Disc Thickness - how to interpret?

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Wp57a
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Minimum Brake Disc Thickness - how to interpret?

Post by Wp57a »

Hi everyone, first post so thanks for having me on here.

The issue of disc thinkess is not new but I can't find a coherent answer to my question so here goes:-

- If you are having brake pads changed and it is found that the discs are below min thickness, then you have the discs changed, no question.
- If the discs are above min thickness then no need to change the discs - just change the pads.

But this means in many cases - that during the lifetime of the new pads, the discs will fall below min thickness.
Possibly - if the discs were nearly at min thickness at the time of the pads change - this will result in the discs being below minimum thickness for almost the whole of the life of the new pads.

So - it surely must be the case that the min disc thickness is set by the manufacturers to allow for the new pads to take the discs below that thickness - this must be the case?

Reason I ask
- I had some new front pads fitted a while ago - the disc thickness was OK.
- Now at service time, the pads are part worn (as expected) and the garage says the discs are now below min thickness so recommend change - not next time new pads are needed, but right now.
- If a service was not due, I would be driving until the pads needed changing.

This suggestion to change now sounds like madness and job creation to me - what do you think?
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by Gibbo2286 »

It sounds like someone is trying to flog you a new set of discs and pads that you probably don't need.

I'll bet if you ask them to tell you the actual 'measured' thickness of of your discs they won't be able to tell you and probably they'll also not be able to tell you what the specified minimum is for your car without going and looking it up.
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by Michel »

I take anything garages say with a pinch of salt, unless it's a couple of chaps on here (Lighty, Wheeler)..

Best way to do it is get a micrometer and measure them yourself, but in most cases, if they look ok, they are ok. What sort of driving do you do? High speed motorway driving, lots of hard stops and braking, or pottering around town slowly? If the former, I'd be more concerned. If mainly the latter, They'll 99.9999% be fine until the next set of pads are needed.

There will obviously be doom-mongers along shortly to say something like "it's brakes, how dare you skimp on them" blah, blah, blah ...
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by GiveMeABreak »

Section 3.5 of the MOT Testing Manual for Passenger Cars states:
Test: The condition and security of brake drums and discs.
Reason for failure: A brake disc or drum in such a condition that it is seriously weakened or insecure.

So they don't test for 'thickness', just on weakness.

Each driver will be different in how often and how hard they use their brakes - all this has a bearing on the wear of the pads and the discs, especially with warping of the discs down to friction and cooling with water on the roads - so a thinner disk may warp considerably more under these sorts of conditions leading to 'juddering'.

The other thing is that if the disc is badly scored, it will affect the new pads anyway.

My C5 goes through a set of front Discs about every 3 sets of pads.

Basically the discs shouldn't go below the manufacturer's safe minimum thickness.

How to measure:
If you don't have a micrometer or other gauge, then you can improvise to get a ball part idea:
  • Use a couple of pound coins and place these flat on either side of the disc.
  • Measure the thickness from outer edge to outer edge of the coins, write the figure down.
  • Remove the coins, place them flat together and measure them.
  • subtract the coins measurement from the previous measurement and that will give you the thickness (albeit non-precise) of the discs.
  • Go to an online parts store and select the correct brake disc for your car.
  • There should be a detailed info section for each dics shown, select that and somewhere you will see the minimum disc thickness.
  • These will vary slightly depending on the brand of discs you have on your car.
If you want to know the original discs type for your car, post up the VIN

Again, others may argue about when to change discs and what thickness is ok, but you've asked the question, so the answer has to be that they should not be below the manufacturer's recommended minimum thickness.
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by Stickyfinger »

Extra disk cooling ?
Image

Measure and decide.
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by CitroJim »

Stickyfinger wrote: 26 Oct 2017, 14:19 Extra disk cooling ?


:lol:
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by GiveMeABreak »

You been photoshopping again?
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by Gibbo2286 »

That looks like a musical instrument, I be it played a good tune. :)

I've never seen one like that but I once got called out to an Alfasud stuck in the middle of a main junction in Hereford, the lady had let the rear pads grind away until one of them fell out and got wedged in the hole in the disc locking up the wheel.
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by CitroJim »

I've seen pictures of badly worn discs shattered...
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by jgra1 »

I remember many years ago, leaving the nice ?town of Petworth on a previous bike, anyone that knows that road, the A272 east, will know it's a nice straight after the slow one way system of Petworth.. attaining as much speed as the old nag would muster, I then braked hard for the right-hander at the end.. something flew out at bullet speed into the hedge in front of me!
the pad had got so worn it self ejected forwards from the caliper ! lol, an odd design really.. think the bike was a 250 honda dream!
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by daviemck2006 »

For all the price that discs are I tend to change them along with pads every time. Tyres and brakes are the two things I am fussy with, any of them fail and you could die.
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by Michel »

daviemck2006 wrote: 26 Oct 2017, 18:52 For all the price that discs are I tend to change them along with pads every time. Tyres and brakes are the two things I am fussy with, any of them fail and you could die.


I think the pads and the discs are the least likely part of the braking system to fail!
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by daviemck2006 »

Michel wrote: 26 Oct 2017, 18:57

I think the pads and the discs are the least likely part of the braking system to fail!


My ex wifes pug 206 had relatively recent discs and pads on the back when she phoned one day and said the car is broken, the back wheel is solid. I went to retrieve it, a large chunk of the disc had broken and jammed locking the wheel solid. Its just as well it happened in town, 1/2 mile from her work.
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by CitroJim »

jgra1 wrote: 26 Oct 2017, 17:23think the bike was a 250 honda dream!


We used to have another name for those :wink: :lol: Something to do with nocturnal experiences of pubescent boys...

One thing I well remember about those particular bikes were their camshafts made of soft cheese!

My old Suzuki bikes (GT500 and GT550) had lethal discs at the front... Rotors made of lovely, bright stainless steel so they always looked lovely and shiny. Indeed they did but the downside is they never worked in the wet :twisted:
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Re: Minimum Brake Disc Thickness - how to interpret?

Post by jgra1 »

ahh yes.. I had a gt500 too Jim ;)
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